Dark Matter is a theoretical form of matter. This doesn’t mean that it’s fictional – we’ve detected it – it just might be different and behave differently to what we expect. Whatever Dark Matter is, it seems to comprise 80% of all the matter in the universe. But in some regions of the universe that is not the case. In one particular galaxy there is only 0.01% ordinary matter; the rest is dark matter, and we still don’t know exactly what this is.